A system's power factor relates to its efficiency because it is a ratio of the system's real power to its apparent power. For example, a non-linear load such as an electric motor presents a substantial amount of inductance to an AC power main. At startup, the input current must thus ramp up from zero whereas the AC voltage across the load continues to cycle with the AC power main voltage. The voltage and current into the load are thus out of phase with each other. A worst case with regard to power delivery occurs when the voltage and current are orthogonal to each other (90° out of phase) such that the power delivery is entirely reactive without any real power delivery. But apparent power is the magnitude of the product of the voltage and current (no phase dependence) such that such a device would continue to burn apparent power although the power factor would be zero. In contrast, a purely resistive load would produce no reactive power component such that the real power and the apparent power are equal, producing an optimal power factor of one. As the reactive power component increases, the power factor drops from one towards zero.
The power factor is thus an important parameter for a switching power converter such as a flyback converter as its transformer inevitably presents a non-linear load to the AC power main. It is thus conventional to implement power factor correction in a flyback power converter. For example, a diode bridge and capacitor may be used to provide a rectified input voltage for a flyback power converter. Despite the rectification, the input voltage will still have a sinusoidal profile for relatively small (and thus inexpensive) input capacitors. To obtain a high power factor, the peak current through the primary winding in the flyback converter's transformer should also have a similar sinusoidal profile. This peak current through the primary winding is proportional to a product of the input voltage and the on time (ton) for the corresponding switching cycle in the flyback converter. The on time is proportional to a ratio of the peak primary winding current to the input voltage. If the peak primary winding current amplitude is tied to the input voltage (as it should be for a high power factor), one can thus readily see that a constant on time achieves a high power factor.
But the resulting constant on time causes an undesirable ripple in the flyback converter's output voltage. In that regard, it can be shown that the output power of a flyback converter is proportional to a square of a product of the input voltage and the on time. Since the input voltage varies sinusoidally, the output power (and hence the output voltage) also varies sinusoidally for constant on time power factor correction. The flyback converter must then use a relatively large output capacitor at the secondary side of the transformer to filter the output ripple, which raises costs. To achieve high power factor correction yet also provide a tightly regulated output voltage, it is thus conventional to use a multi-stage approach such as by processing the output voltage from the flyback converter through a DC-DC switching power converter. But such multi-stage approaches are expensive in terms of the extra components and increased control complexity. It is thus more economical to use a single-stage flyback converter architecture. But such users are then left with a choice of an improved power factor with a poorly-regulated output voltage or a tightly-regulated output voltage with a decreased power factor.
Accordingly, there is a need in the art for single-stage flyback converters with power factor correction and reduced output voltage ripple.